"Safety valves" and "safety relief valves" (also referred to herein as simply "valve" or "valves") monitor and relieve abnormal or dangerously high pressures in systems that handle, generate, store, contain, confine, carry, transfer or transport pressurized fluid (material in liquid or gaseous states). Under normal operating pressures, the valves remain closed and sealed. When the pressure of the fluid exceeds a predetermined maximum system pressure, the valve opens to dispel fluid and thereby relieve pressure. When installed, a valve is in direct contact with the pressurized fluid. The pressure of the fluid applies a force to the surface of the valve's disc. To resist the normal pressure of the fluid, the valve disc is "loaded" to create an opposing force to the pressure force of the fluid. The load forces the valve disc into a valve seat and thereby creates a seal. The load is adjusted to set a force equal to a predetermined maximum system pressure so that the valve disc is displaced or "lifted" from the seat and the seal broken to allow fluid to flow only when the system pressure exceeds the allowed maximum system pressure.
There are a variety of methods for operating a safety valve or a safety relief valve. Some are purely mechanical; some are electro-mechanical or magnetic. Under codes of the American Society of Mechanical Engineers (ASME), all are required to have a purely mechanical method of loading the valve for at least backing up the operation of the valve. The common, industry-wide method involves loading the valve with a force generated by a compressed spring.
Safety valves and safety relief valves are used in an enormous variety of situations, wherever there exists danger from over-pressurized fluid. Safety valves are used with systems such as boilers and steam generators, and safety relief valves are used in systems such as manufacturing processes, pipes for transferring or transporting fluids, and other types of vessels, containers or channels for carrying or storing fluid under pressure. In addition to their construction, use and operation of safety valves and safety relief valves are mandated and regulated by industry and, sometimes, government guidelines that generally follow or adopt the American Society of Mechanical Engineers (ASME) code, particularly Sections 1 (for safety valves) and 8 (for safety relief valves).
Knowledge of whether a valve is leaking or flowing is important. A flowing condition indicates that the system has been over-pressurized, possibly requiring any number of actions such as shutting down the system to avoid or to mitigate damage. A leaking condition may indicate one of two possibilities. First, a leak may result from the system being operated close to the relief point. Detection of leaking thus serves as warning to take corrective action to reduce pressures before fluid is relieved. Second, a valve that leaks may be defective or malfunctioning, requiring service or replacement. Further, where fluid leaks out of the system, product--and money--is unnecessarily lost.
Aside from actually witnessing a fluid escaping from a valve, monitoring the volume of fluid in a system may be the simplest method for knowing whether a safety valve or safety relief valve is leaking or flowing. Where, however, multiple numbers of safety valves or safety relief valves are installed in a system, each set at the same pressure point, it becomes impossible to isolate which valve is leaking. Without being able to detect which valve is leaking, all the valves must be serviced or replaced. Therefore, a leak detection mechanism is desirable.
A leak and flowing detection mechanism is also desirable in automated production facilities or anywhere valves have been remotely installed, such as modern petroleum refineries and petrochemical plants that are operated from remotely located control centers for reasons of safety and economy. Opening of a safety relief valve may be the only way of discovering or monitoring unsafe pressure conditions. Safety and safety relief valves for these facilities are, ideally, equipped with transducers for detecting the opening of a safety relief valve and communicating the detection to the central control center. As fluids handled in these facilities are often volatile, the transducers, as well as the system for communicating the transducer output to a central monitoring station, use very low voltages to reduce the risk of sparks that might ignite the fluid.
For many years, the preferred method of detecting valve openings caused by excessive fluid pressure have been switch devices such as so-called micro-switches. Switches are mechanically coupled to a moving part of the valve, usually the rod-like spindle or valve stem, that moves or is "lifted" with a linear displacement of the valve disc along the axis of the spindle. These switches have proven to be unsatisfactory in the illustrated situations for several reasons. The switches have a substantial "dead zone" between an open contact and a closed contact position, making them difficult to adjust to detect small spindle movements characteristic of leaking. If installed in the valve in positions at which they may be able to detect small movements in the spindle characteristic, a full lift condition will damage them. Therefore, when installed so that they are not damaged, substantial movement of the spindle is required before being actuated or switched, making them suitable only for detecting flowing conditions in the valve when the valve is at or near "full lift". The switches require relatively high-voltages, making them unsuitable for applications in which voltages in the sensor must be kept low. They are cumbersome to set or adjust in the field as they tend to be intricate. Once set, the switches tend to be easily tampered with and easily become misadjusted by mechanical vibrations common to many applications. Even if they do retain their proper settings, the shock of a release with full-lift of the valve disc frequently damages the switches. Finally, the switches are sensitive to the environment, requiring protective housings to be installed around the switching mechanism.